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SSD/Hard Disk Hybrids Bridge Storage Divide

Solid-state is creeping into every nook of the storage ecosystem. The next frontier could be the rebirth of hybrids. Case in point is a startup that combines SSDs and traditional disks to maximize both performance and capacity.

Solid state is colonizing every corner of the storage ecosystem, from internal server caches to standalone SSD arrays. Heretofore, solid state has been treated as a distinct storage layer. However, a hybrid approach blurs the lines between electronics and magnetics by closely pairing solid-state and mechanical disk storage to achieve optimal price/performance balance.

The goal of dramatically expanding access to solid state's performance-enhancing benefits without breaking the bank, while simultaneously solving some of the biggest storage problems in virtual servers, is the dream of Enmotus, a stealthy startup that opened its kimono at last fall's Server Design Summit by demonstrating what it calls MicroTiering technology. As CEO and co-founder Andy Mills puts it, the company's "whole focus is an affordable tiered hybrid storage, at a lower price point than Fusion-io." Translated: solid-state cache-like performance at the price point of a commodity hard drive.

The trade-off between performance and capacity is a fundamental principle of digital storage. Regardless of the physical means of storing bits, system designers can have speed or size, but not both. It's the tenet undergirding the canonical multilevel cache architecture used in all modern processors, with a three (or more)-deep hierarchy for data access where fetches from each successive level take longer.

The same phenomena is now manifest in the realm of persistent storage as flash-based solid-state devices have finally scaled to the point where they're feasible replacements for magnetic disks in many situations. Here the yin and yang between performance and capacity is particularly striking. The sweet spot of for SSDs is currently 256 Gbytes versus 2 Tbytes for SATA hard drives (8 times more). In return, SSDs provide two to three orders of magnitude higher IOPs and about a hundred-fold reduction in write latency.

But engineers hate trade-offs and constantly seek design optimizations to provide the best of both worlds, at least under normal circumstances.

Enter hybrid storage, where Mills claims MicroTiering delivers nearly identical performance--within 10% on real-world applications that haven't been specifically tuned for local solid state caching--to a caching-plus-HDD or all-SSD implementation.

MicroTiering gets its name from the fact that it gloms storage volumes into a cohesive union of SSD and HDD that to the outside world look like a single device. But unlike server-side caching products made famous by Fusion-io that simply front-end HDDs with a veneer of flash, Mills emphasizes that in MicroTiers the SSD acts as primary storage. The MicroTiering software automatically migrates "hot," active data to the SSD tier and "cold," inactive data to the HDD. The scheme sounds a lot like Apple's Fusion Drive, but when offered this comparison, Mills hastened to point out that MicroTiers work at the block layer, while Fusion operates on files.

Conventional wisdom, perhaps, but as my colleague Howard Marks points out, not true. Fusion drives do indeed operate below the file system as detailed by these tests showing that when reading just the first megabyte from 300 100-Mbyte files, each came out of the SSD while subsequent reads for the remainder of a file came from disk. Still, Fusion is a bundled device that only works on a select set of proprietary systems. In contrast, Enmotus builds MicroTiering into a an ACHI-compliant PCIe storage adapter that works on any server and with virtually any SAS or SATA device, much like a conventional RAID HBA.

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